The purposes of a formation of a chromate conversion coating on the surface of galvanized metal are to provide corrosion resistance, improve adhesion of coatings and for aesthetic reasons. Chromate passivation of a galvanized steel surface is done to provide corrosion resistance and for aesthetic reasons on materials which are not to be painted. A bulky, white corrosion product may form on an unprotected bright zinc surface when it becomes wet. This corrosion product is a mixture of zinc carbonate and zinc oxide or hydroxides resulting from zinc oxidation. The conditions producing the "humid storage" stain (so called white rust) most frequently occur in shipment and during storage, especially when daily temperature variations cause atmospheric water vapor to condense on a zinc surface. Likewise, black stains form on unprotected Galvalume. Galvalume is a trademark of the Bethlehem Steel Corporation for a zinc-aluminum galvanized coating over steel.
Chrome based passivation treatments are applied to galvanized metals and Galvalume to provide both long term and short term corrosion protection. A chromate treatment is typically provided by contacting galvanized metal with an aqueous composition containing hexavalent and trivalent chromium ions, phosphate ions and fluoride ions. Growing concerns exist regarding the pollution effects of the chromate and phosphates discharged into rivers and waterways by such processes. Because of the high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate processes require extensive waste treatment procedures to control their discharge. In addition, the disposal of the solid sludge from such waste treatment procedures is a significant problem.
Attempts have been made to produce an acceptable chromate free conversion coating for passivation of galvanized metal. Chromate free pretreatments based upon complex fluoacids and salts or metals such as cobalt and nickel are known in the art. U.S. Pat. No. 3,468,724 which issued to Reinhold discloses a composition for coating ferriferous and zinc metal which comprises a metal such as nickel or cobalt and an acid anion selected from the group sulfate, chloride, sulfonate, citrate, lactate, acetate and glycolate at a pH of from 0.1 to 4.